Lightning arrester and gap unit therefor

ABSTRACT

A lightning arrester particularly adapted for extra high voltage AC systems and high voltage DC systems comprising a series of arc gap units and coil assemblies for moving the arcs outwardly. The gap units and coil assemblies are connected in series between a line of the system to be protected and ground without the intervention of series valve blocks. The gap units develop arc voltages greater than the system line to ground voltage so that the gaps can interrupt the power follow current, but the total of the gap voltage and coil assembly voltage does not exceed the allowable protective level at any time. The gap units are designed so that the arcs flutter so as to minimize localized overheating and erosion of the gap plate material.

I Unlted States Patent [151 3, Kalb [451 July 18, 1972 LIGHTNING ARRESTER AND GAP FOREIGN PATENTS OR APPLICATIONS UNIT THEREFOR 656,460 8/1951 Great Britain ..317/74 [72] Inventor: John W. Kalb, Medina, Ohio 1,064,556 12/1953 France ...3l7/73 Assigneez The Ohio Brass p y Mansfield, 487,602 11/1929 Germany ..317/74 Ohio I Primary Examiner-J. D. Miller [22] Filed: Jan. 7, 1970 Assistant Examiner-Harry E. Moose, Jr. [21] APPL No: 1,186 Anorney-Bosworth, Sessions, Herrstrom & Cain [57] ABSTRACT [52] U.S. Cl ..3l7/6l, 313/153, 317/70,

317 74 A lightning arrester particularly adapted for extra hlgh voltage [51] Int. Cl. ..H02h 9/06 AC Systems and high Voltage DC systems comprising a Series [58] Field of Search .317/61, 70, 73, 74; 200/144; f are g p un n il a em lies for moving he arcs out- 313/231, 161, 153, 156 wardly. The gap units and coil assemblies are connected in series between a line of the system to be protected and ground References Cited without the intervention of series valve blocks. The gap units develop are voltages greater than the system line to ground UNITED STATES PATENTS voltage so that the gaps can interrupt the power follow cur- 3,513,354 5/1970 Sakshang ..317/61 rent, but the total of the gap voltage and coil assembly voltage 3,019,367 1/1962 Kalb 317/70 X does not exceed the allowable protective level at any time. 1,273 9/1964 St ts n t al--. 20 /144 X The gap units are designed so that the arcs flutter so as to 3,354,345 67 t ts n 317/73 X minimize localized overheating and erosion of the gap plate 3,414,759 12/1968 Connell et al 317/70 X material 3,378,722 4/1968 Osterhout et al.. 317/73 X 2,566,895 9/1951 Kalb ..313/243 X 16 Chins, 11 Drawing Figures iALLOWABLE PROTECTIVE LEVEL COIL CURRENT I fiTOTAL VOLTAGE I 1 I t col". avAL'vE BLOICK VOLTAGE VALVE ELOCK ICURRENT TIME MICRO SECONDS VOLTAGE KV PATENTED JUL] 8 I972 SHEET 1 OF 4 iALLOWABLE PROTECTIVE LEVEL |2oo {we VOLTAGE! CREST -J soov COIL CURRENT IBOO- I f \3 nzoo- TOTAL voumen;

/ 00:". &VAL:VE BLCIDCK VOLTAGE VALVE BLOCK CURRENT TIME MICRO SECONDS LINE I NV ENTOR.

JOHN W. K ALB BY 6W1, 12W

A T TORNE Y6 PATENTEDJULWMZ 3,678,340

sum u or 4 FIG. 9 FIG. /0

INVENTOR JOHN W. KALB AT TORNEKS LIGHTNING ARRESTER AND GAP UNIT THEREFOR BACKGROUND OF THE INVENTION from unduly high voltages such as result from lightning strokes and switching surges in transmission lines.

The invention is particularly concerned with devices of this type that are designed for extremely severe service as in high voltage direct current transmission lines and extra high voltage alternating current transmission lines. These devices find their greatest usage in the protection of station equipment, such as transformers, against unduly high voltages that may result from lightning strokes striking a transmission line and related equipment or from transmission line surges that may result from opening or closing of switches, circuit breakers, or the like. The devices are hereinafter referred to as lightning arresters" since that is the usual term applied to them in the trade, but it is to be understood that they are useful wherever protection against unduly high voltages is required for electrical equipment and wherever it is necessary to open or close circuits at high voltages.

Lightning arresters have been used in the protection of electrical equipment for many years. At low voltages it was only necessary to provide a spark gap in a circuit leading from the line to be protected to ground. A high voltage surge resulting from a lightning stroke would jump the gap, effectively grounding the line and protecting equipment connected to the line from the high voltage of the lightning stroke. As soon as the lightning stroke was over the arc across the gap would stop or be extinguished, no current would flow from the line to ground and the system would be back in normal operation. As voltages became higher and higher, simple spark gaps were not enough because the arc established by the lightning stroke or other high voltage surge would persist after the stroke had ceased and a current (known as follow current") would flow to ground under the influence of the line voltage. If follow current is permitted to flow to ground for any appreciable length of time, not only will the lightning arrester itself be damaged, if not destroyed, buy also the resulting very nearly short circuit conditions will cause the circuit breakers on the system to open and the system will be put out of operation.

Heretofore, and with alternating current transmission lines having voltages as high as 500 kv, satisfactory operation has been obtained with lightning arresters of the type shown in my prior U.S. Pat. No. 3,019,367, issued Jan. 30, 1962, and other lightning arresters embodying what are known as current limiting arc gaps connected in series with non-linear resistors known as valve blocks". See also the Hazen U.S. Pat. No. 3,443,149, issued Jan. 6, 1969, which discloses a current limiting gap embodying porous ceramic plates. in these devices a high voltage surge, such as a surge resulting from a lightning stroke or switching surge, jumps the arc gaps at a voltage low enough that the line voltage does not become high enough to damage the equipment connected to the line. The non-linear resistor has a fairly low resistance at high voltages and thus the surge is diverted harmlessly to ground. The gaps are constructed so that the arcs are elongated and energy is absorbed from the arcs, the voltage drop across the arcs increases greatly and the current flowing across the gaps is thereby limited. The increase in voltage drop across the arcs reduces the voltage across the valve blocks, increasing the resistance of the valve blocks. Ultimately the voltage of the arcs plus the voltage of the valve blocks reaches a valve such that the power follow current is diminished appreciably and the arc is extinguished, usually within less than a half cycle.

As mentioned above, devices of this general character have been utilized with success in alternating current transmission lines carrying voltages up to about 500 kv and with direct current lines of lower voltage. However, with some DC lines and extra high voltage AC transmission lines, it sometimes becomes difficult to meet requirements with these devices,

either for the reason that they would furnish insufficient protection to the equipment or for the reason that the arresters themselves would be short-lived under service conditions and might be damaged if not destroyed by a succession of surges.

With transmission lines of this character the most serious difficulties occur with switching surges rather than lightning strokes, the reason for this being that switching surges ordinarily are of much longer duration than lightning strokes. Thus, with a conventional lightning arrester embodying a valve block and a current limiting gap in series, the duration of the lightning stroke is so short that no significant voltages are developed across the gaps while the lightning stroke is going on and the particular problem of the arrester is to shut off the follow current promptly. The voltage across the arrester during the follow current does not exceed the insulation strength of the equipment that is being protected.

Mathematical analysis shows that for ordinary high voltage AC lines, for example 69 kv, the usual arrester embodying current limiting gaps and valve blocks in series is entirely adequate to discharge the switching surges without the development of voltages across the arrester that are greater than the allowable protective level. However, this is not the case in extra high voltage AC systems. For example, in a typical 765 kv system, the desired performance characteristics are that the voltage across the arrester shall not exceed 1,200 kv. Analysis shows that with a typical arrester embodying valve blocks and current limiting gaps and with the high switching surge currents that occur in these relatively low impedance transmission lines, the arc voltage that builds up across the current limiting gaps as the arcs in the gaps increase in length and absorb energy, added to the 1R drop across the valve blocks will exceed the permissible 1,200 kv. Thus, simple extension of the design of present lightning arresters by inclusion of more gaps and more valve blocks is not practical for extra high voltage AC transmission lines.

SUMMARY A general object of the present invention, therefore, is to provide lightning arresters that are suitable for protection of extra high voltage alternating current transmission systems and that are also suitable for protection of direct current transmission systems. More specific objects include the provision of lightning arresters having the ability to protect equipment from switching surges of relatively long duration as well as from lightning strokes; the the provision of such lightning arresters that can be manufactured at reasonable cost; and the provision of such arresters that are durable andconsistent in operation over long periods of time.

The invention contemplates the provision of lightning arresters embodying current limiting gaps but without any series valve blocks. The arresters, instead, employ current limiting gaps in series with a coil for moving the arcs in the gaps outwardly in the usual manner, there being a non-linear resistor such as a valve block in shunt with the coil. The time constant of the coil is correlated with the rate of movement of the arc and the development of the voltage across the gaps so that the impedance of the coil and, accordingly, the voltage drop across the. coil and its parallel connected valve block is reduced to a very low value before the arc voltage has been greatly increased, the maximum arc voltage being sufficiently below the designed protection lever that the arc voltage plus the voltage drop across the coil will be below that level, and the arc voltage being sufficiently greater than the line voltage that the arc gaps by themselves, without help from a series valve block, can interrupt the power follow current. Another aspect of the invention relates to the specific design of the arc gaps that contributes to their ability to interrupt DC as well as AC and enables them to withstand severe operating conditions and maintain reasonable durability and life. This is accomplished by providing for rapid movement or fluttering of the arcs in most of the areas where energy absorption takes place so as to minimize localized overheating and erosion of the gap material.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings:

FIG. 1 is a side elevation with parts broken away of the gap, coil and valve block elements of a lightning arrester embodying a preferred form of the present invention, removed from their conventional housing for convenience of illustration;

FIG. 2 diagrammatically illustrates the electrical connections of the arrester of FIG. 1;

FIG. 3 is a chart indicating a typical relationship of certain voltages and currents during the operation of the arrester;

FIG. 4 is a top plan view of two of the gap units employed in the arrester;

FIG. 5 is a bottom plan view of one of the gap units of FIG.

FIGS. 6 and 7 are vertical sectional views on an enlarged scale through the gap 4; of FIGS. 3 and 4 in assembled position, the sections being taken along lines 6-6 and 7-7 of FIGS. 4 and 5;

FIG. 8 is a sectional detail taken as indicated by the line 8 8 of FIGS. 4 and 5, but showing the plates assembled;

FIG. 9 is a top plan view of the top end plate for a pack of gap plates;

fig. 9 s a op pla view oftte toppen plteefor a paakkoog FIG. 10 is a bottom view of the bottom end plate for a pack of gap plates; and,

FIG. 11 is a section of one of the coil and non-linear resistor assemblies used in the arrester assembly shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The general arrangement of a preferred embodiment of a lightning arrester embodying the present invention adapted for high voltage DC service or extra high voltage AC service is illustrated somewhat diagrammatically in FIG. I of the drawings, the conventional insulating housing for the arrester being omitted for convenience ofillustration. As there shown, the arrester I0 is interposed between a line and ground. The function of the arrester is to divert surges, whether resulting from lightning or switching surges or the like, harmlessly to ground, thus preventing voltages in excess of the insulation value of the equipment the arrester is designed to protect from reaching the equipment, and then after the surge has ceased, very promptly to shut off the power follow current. In the present example the arrester is designed to protect the apparatus in a'765 kv AC system against surge voltages in excess of 1,200 kv.

As shown in FIG. I, the arrester I0 is made up ofa series of gap packs 11, each pack comprising an upper end plate 12, a series of intermediate plates 13 and a lower end plate 14. As will appear below, there are electrodes providing gaps between the lower surface of the upper end plate 12 and the upper surface of the adjacent intermediate gap plate 13, between the oppositely facing surfaces of the intermediate gap plates and between the lower surfaces of the lowermost intermediate gap plate 13 and the upper surface of the lower end plate 14. Each pair of opposing surfaces and the electrodes incorporated therein constitutes a gap unit. In the embodiment shown there are five intermediate gap plates, making a total of six gap units and six gaps in series in each gap pack. There are sufficient gap packs of this type in the arrester so that the sparkover voltage of the arrester will be slightly less than l,200 kv. The number of gap units in a pack and the number of packs in an arrester, of course, can be varied in accordance with the design requirements, the desired sparkover voltage of the arrester and the sparkover voltages of the individual gaps. An idea of the size of a typical arrester can be obtained, however, if a typical sparkover voltage of 6 kv for each gap is assumed. Each gap pack, then, will have a sparkover voltage of 36 kv and 33 gap packs and associated coils in a stack about 15 feet high will be required to provide the desired 1,200 kv sparkover,

The gaps are of the type in which the arcs are extended, and ultimately extinguished, by means ofa magnetic field which is generated by coil assemblies l6 that are disposed at the top and bottom of the arrester and between adjacent gap packs. A plurality of coils are required in order to provide a sufficiently uniform magnetic field throughout the length of the arrester so that the arcs will move in their respective gaps with a reasonable degree of uniformity. Each coil assembly embodies a coil 17 and a valve block 18 connected in shunt with the coil.

The wiring arrangement of the arrester, with grading resistors and the like omitted for convenience of illustration, is shown in the diagram constituting FIG. 2. As there indicated, the arrester embodies a top coil assembly 16 that is connected to the line, and alternately disposed gap packs I1 and coil assemblies 16; the arrester terminating in the bottom coil assembly 16 that is connected to ground. Each coil assembly l6, as noted above, comprises a coil 17 and a non-linear resistor in the form ofa valve block 18 connected in shunt therewith, and each gap pack comprises a plurality of gap units indicated diagrammatically at 20 and connected in series. The gaps are of the current limiting type that develop substantial gap voltage as the arcs are extended by the effect of the field created by the several coils l7. Ultimately the total gap voltage reaches such a value that the arcs are extinguished.

The valve blocks 18 are provided to protect the coils 17 by introducing a parallel, low impedance path for the surge current, this being necessary since the surges have very steep wave fronts and, in fact, may be considered as square waves so that the initial impedance of coils 17 is very high. The valve blocks 18, on the other hand, like the conventional valve blocks that are utilized in series with the gaps of prior types of lightning arresters, offer relatively low impedance to the initial surge, but the impedance of the valve blocks increases greatly as the voltage applied to them decreases. The design of the coils, the valve blocks and gap units are all correlated, as noted above, so that in the event of a lightning stroke, switching surge or the like, the gaps break down before the line voltage exceeds the predetermined value and the gaps and the parallel-connected non-linear resistor and coil assemblies connected in series with the gaps provide a low impedance path to ground.

Initially, very little current flows through the coils, most of the current flowing through the valve blocks with the result that the arcs are not greatly extended. However, the impedance of the coil is reduced after the initial very steep wave front of the surge has passed and more of the current begins to flow through the coils, thus building up a magnetic field that extends the arcs in the gap units. As time progresses, the voltage across the valve blocks becomes lower and their impedance increases and the total voltage drop across the gaps 20, referred to herein as arc voltage, increases greatly. The coils, gap units and resistors are designed so that regardless of the rapid and substantial increase in the arc voltage as the arcs move outwardly, the total arrester voltage does not exceed the permitted voltage (allowable protective level), for the reason that the impedance of the parallel connected coil and resistor assemblies decreases as the arc voltage increases. The result is that the equipment connected to the line is properly protected and the follow current is very promptly shut off after the termination of the surge, primarily by the extension of the arcs in the gap elements.

The curves constituting FIG. 3 illustrate various currents and voltages plotted against time in a typical arrester made according to the present invention and designed for use with a 765 kv AC line. The events that take place in a lightning arrester when a system which the arrester is protecting is subjected to a lightning stroke, a switching surge or other overvoltage condition are very complex. However, these curves are thought to represent conditions in the arrester with sufficient accuracy to furnish a basis for an explanation of the mode of operation of the arrester.

As stated above, the arrester for which these curves were drawn is designed for a 765 kv AC system. The crest of the rated normal line to ground voltage in such a system is 625 kv. The allowable protective level is assumed to be 1,200 kv; in other words, the transformers and other equipment associated with the transmission line Should not be subjected to voltages in excess of 1,200 kv. A rather severe switching surge of 1,700 kv is also assumed. With a 765 kv system protected against 1,200 kv, and with a 1,700 kv switching surge, there must be a 500 kv drop across the line surge impedance if no more than 1,200 kv discharge voltage is to appear across the arrester. It is assumed that the line has a 275 ohm surge impedance, a value that is typical of lines of this character. This would require a flow of slightly more than 1,800 amperes through the arrester after sparkover if the line voltage is to be dropped from 1,700 kv to 1,200 kv. FIG. 3, therefore, is also based on the assumptions that an 1,800 ampere surge of square wave front is discharged through the arrester and that the arrester has sparked over at time zero.

The total sparkover voltage of the series gaps is about 1,200 kv but immediately upon sparkover the impedance of the gaps of current through the coils to a very low value and the IR drop across the gaps is initially so low as to have a negligible effect on the operation of the arrester. This is indicated by the curve of gap arc voltage shown in FIG. 3, which is practically zero at time zero. The impedance of the coils 17 to the very steep wave front is so great that the current carried by the coils at time zero may also be considered to be zero. Based on the assumed 1,800 ampere surge current, the current through the shunting valve blocks (non-linear resistors) 18 is 1,800 amperes. As the inductance of the coils is overcome, however, the flow shown increases rapidly, as shown by the curve, while the current through the valve blocks 18 correspondingly decreases. In other words, the reduction in the impedance of the coils reduces the voltage across each parallel connected coil and valve block assembly, resulting in an increase of the resistance in the non-linear valve blocks and a corresponding reduction in the flow of current through them. Thus, as shown by the curves, after about 500 microseconds have elapsed nearly all of the current is flowing through the coils, since the impedance of the coils is largely the low resistance of the copper, and almost no current is flowing through the valve blocks. Thus, while the resistance of the valve blocks becomes substantial, the total IR drop across the coil and resistor elements which is initially great enough to limit the current through the arrester to a reasonable value (about 1,800 amperes in the example given), ultimately becomes very low, as shown by the curve.

While the coil and valve block voltage is being reduced, the flow of current through the coils creates a magnetic field which causes the arcs to extend, the gap arc voltage increasing greatly, as shown by the curve, while the coil and valve block voltage is decreasing. Thus, as the broken line curve illustrates, initially the total voltage across the arrester is substantially the same as the coil and valve block voltage. The total arrester voltage falls immediately after sparkover because initially the coil and valve block voltage falls off faster than the gap arc voltage increases. However, this drop in arrester voltage ceases as the gap arc voltage increases and after a little more than 400 microseconds the voltage across the arrester depends almost entirely on the gap arc voltage; this increases rapidly as shown to a maximum value that, while high, is still less than the allowable protective level of 1,200 kv.

It will be noted that the maximum total voltage across the arrester is made up of the gap arc voltage of about 1,100 kv and the coil and valve block voltage of a few kv. This mode of operation is made possible by the correlation of the design of the gap units and the coil and valve block assemblies, with particular reference to the correlation of the coil inductance (and hence the rate at which the impedance of the coils decreases) and the rate at which the gap arc voltage increases. If the coil and valve block voltage did not decrease with time, but instead remained primarily a function of current as is the case with series valve block in conventional arresters, the allowable protective level would be exceeded when the gap arc voltage had approached its maximum value. However, with arresters made according to the present invention the arrester voltage remains below the allowable protective level even though the switching surge may persist for several milliseconds (several thousand microseconds). Even with surges of extended duration the valve blocks are not damaged since very little current flows through them after the initial wave front has passed. The arresters, therefore, have the ability to discharge switching surges safely without damage to the equipment and as soon as the switching surges have been dissipated the arcs are promptly extinguished without requiring the power system voltage to go to zero, since the arc voltage exceeds the normal Iine-to-ground voltage. This characteristic makes the arresters effective in DC systems.

The curves of FIG. 3 have been drawn with particular respect to protection against switching surges since these, because of their duration, are the most damaging surges and most difficult to control and protect against in extra high tensionsystems. The arrester, however, is effective against surges created by lightning strokes, the operation being similar except that the maximum voltages and the instantaneous values of the current are usually much greater. As with a switching surge, initially the current flows through the valve blocks and the gaps, but lightning strokes are of very short duration. ordinarily of the order of a few microseconds, and the valve blocks can withstand the heavy current flow for this duration without damage. As soon as the steep wave front has passed, current begins to flow in the coils, the gap arc voltage increases and in about 500 microseconds the gap arc voltage exceeds the normal line-to-ground voltage and the power follow current is shut off by the gaps.

This mode of operation requires gap units of a novel and improved form, having more accurate sparkover voltage than prior gaps, controlled movement of the arcs and slower build up of the maximum arc voltages than prior types of current limiting gaps. The gap units should also be substantially free from restriking of the arcs between the electrodes, and have the ability to develop very high are voltages since the gap units have almost the entire duty of interrupting the power follow current without assistance from valve blocks. The gap units also must have the ability to withstand repeated and prolonged strokes and surges without damage and without impairment of their interrupting ability. A preferred form of gap unit for obtaining these results is illustrated in FIGS. 4 to 8 of th drawings.

As shown in FIG. I, the gap stacks 11 are made up of stacked porous ceramic plates that support the electrodes for the gaps. The plates are preferably in the form of circular discs. FIG. 4 illustrates the upper surfaces of two of the intermediate plates 13, these being identified as plates 13a and 13 b. The plates are shown separated from each other, but when stacked they define arcing chambers between them as described below. FIG. 5 illustrates the under side of plate 13a which cooperates with the upper side of plate 13b, shown in FIG. 4, to provide an arcing chamber. The peripheral portions 22 and 23 of the plates interengage (see FIGS. 6 and 7) to retain the plates in alignment and with proper spacing between the opposed surfaces. The arcing chambers between pairs of intermediate plates and between the upper end plate 12 and the uppermost intermediate plate 13 and between the lower end plate 14 and the lowermost end plate 13 are identical. Therefore, only the chamber between plates 13a and 13b is described herein.

As shown in FIG. 4, the upper surfaces of the plates 13a and 13b are each provided with electrodes 24 and 25, each pair of electrodes and associated arcing chamber constituting one of the gap units 20 diagrammatically indicated in FIG. 2. The electrodes are preferably composed of copper as is conventional and approach each other closely to form a sparkover region 26. From the sparkover region the electrode edges diverge throughout curved portions 27 and finally terminate in substantially straight portions 28 that extend approximately to the periphery of the porous ceramic plates. As explained below, after sparkover the ends of the arc move outwardly along the diverging surfaces 27 and ultimately the ends of the arc travel along the substantially straight portions 28 nearly to the periphery of the discs unless the arc is previously extinguished.

The electrodes are secured in place by an appropriate adhesive such as an epoxy adhesive and connections between the series connected gap units are made by hollow copper rivets 30 and 31 for the electrodes 24 and 25 of plate 13a, respectively. In the drawings, the electrode 25 in each gap is displaced in the clockwise direction from the electrode 24. Successive gaps in each stack are angularly displaced. 1n the embodiment shown the displacement is 120, the electrodes 24 and 25 and the sparkover region 26 of plate 131) being displaced 120 in the clockwise direction from the electrodes 24 and 25 and the sparkover region 26 of plate 13a.

With this arrangement, rivet 31 extends downwardly from electrode 25 of plate 13a through plate 13a and makes electrical contact and is secured to electrode 24 of plate 13b. Rivet 32 extends downwardly from electrode 25 of plate 1312, through plate 13b and is secured to and makes electrical contact with an electrode (corresponding to 24) on the plate 13 disposed immediately beneath plate 131;, and the rivet 30 of plate 13a extends upwardly through the plate 13 disposed immediately above plate 13a and into contact with an electrode (corresponding to electrode 25) on that plate. Thus, in each stack, the electrodes and gaps are disposed in a helix with the common axis of the discs in the stack as the axis of the helix. These arrangement is generally similar to that shown in my prior U.S. Pat. No. 3,019,367, issued Jan. 30, 1962.

As is shown in FIGS. 6 and 7, the electrodes are spaced slightly from the adjacent plates, it being undesirable to have the copper electrodes in direct surface contact with the porcelain. The desired spacing is accomplished economically and accurately by simply applying to each side of each copper electrode 24 and 25 a layer 33 of insulating tape having the outline shown in FIG. 4 and provided with a pressure-sensitive adhesive on one side thereof to secure the layer to the electrode. The electrodes are cut-away in a known manner as shown at 34.

The upper surfaces of the plates 13a and 1312 are also recessed as indicated in FIGS. 4, 6 and 7 to receive grading resistors 35, a capacitor 36 in parallel with resistors 35, and the elements ofa pre-ionizing gap 37 and an associated resistor 38 and capacitor 39. The under surface of each plate is also recessed as shown at 41 in FIGS. 5, 6 and 7 to provide space for these parts. The grading resistors and the pre-ionizing gaps associated with each plate are important to provide accurate control of sparkover and can be constructed in a manner known to those skilled in the art. Hence, these elements will not be discussed further herein.

As mentioned above and as shown particularly in FIGS. 6 and 7, the plates 13a and 13b define between them an arcing chamber 42 in which the electrodes are disposed. Arcing chamber 42 is defined by the opposed surfaces 43 and 44 on the lower surface of plate 13a and the upper surface of plate 13b, respectively. In the embodiment shown, the distance between the opposed surfaces 43 and 44 is uniform throughout most of the area of the plates but, if desired, a greater distance can be provided between the surfaces at the sparkover region, the spacing then gradually being reduced in the direction of the arc travel. A spacing is selected that will permit the desired movement of the are under the influence of the magnetic field produced by the coils l7 and also will result in increasing arc voltage as the arc is extended.

In order to obtain the desired fluttering action of the arc to prevent localized overheating and damage to the plates in the event of a prolonged surge, fingers 45 and 46 are provided in the arcing chamber. These fingers extend radially inwardly from the periphery of the arcing chamber, and periphery being closed by the interfitting peripheral portions 22 and 23 of the plates 13a and 13b throughout the circumferences of the discs except for the portion adjacent the recess 42. The longer fingers 45 project inwardly toward the center of the discs in the zones opposite the sparkover region 26 and the curved portions 27 of the electrodes24 and 25, while the shorter fingers are disposed in the zones opposite the fairly straight portions 28 of the electrodes 24 and 25. The projections together subtend substantially more than of the peripheries of the plates. It will be noted that fingers 45a and 45b, at the ends of the series of longer fingers 45 extend rather close to the electrodes 24 and 25, as do the shorter fingers 46a and 46b.

The inwardly extending fingers 45 and 46 are provided by forming the upper surface of plate 13b with axially extending projections as shown in FIGS. 6, 7 and 8 that fit into recesses 47 and 48 in the under surface of the plate 13a. The fingers 45 and 46 are each surrounded by small troughs 49 and 50 that increase the distance between the plates 13a and 13b and therefore the thickness of the arcing chamber in the regions immediately surrounding the fingers. The space between the top of each finger 45 and 46 and the bottom of the corresponding recess 47 and 48 and the peripheral portions of the discs are sealed by an appropriate cement as shown at 51.

As noted above all of the gap units 20 are identical; the gap unit at the top of each stack being formed between the upper surface of the uppermost plate 13 and the under surface of the top end plate 12, a top view of end plate 12 being shown in FIG. 9. The under surface of end plate 12 (not shown) has a configuration like the under surface of the plate 13a and as shown in FIG. 5. A hollow rivet 54 extends from an electrode corresponding to electrode 24 mounted on the upper surface of the uppermost plate 13 through the plate 12 into electrical contact with a conductive clip 55 that makes contact with the bottom ofa clip56 on the under side ofthe coil assembly 16.

At the lower end of each stack there is a lower end plate 14, a bottom view of one of which is shown in FIG. 10. The upper surface of each lower end plate (not shown in the drawings) has a configuration identical with the upper surfaces of the plates 13 and cooperates with the bottom surface ofthe lowermost plate 13 in the stack to provide a gap unit 20, electrodes 24 and 25 being mounted on the upper surface of the lower end plate. The bottom of plate 14 is provided with a conductive clip 58 that is connected bya rivet 59 to an electrode corresponding to the electrode 25 on the upper surface of the plate. This clip engages a clip 60 on the top of the coil assembly 16 at the bottom of the stack and immediately below the lower end plate.

The coil units 17 shown in FIG. 11 comprise a spool-shaped body member 62 of insulating material having flanges 63 and 64 which define an annular recess 65 in which the coil 17 is wound. The valve block 18 is disposed in a central recess 67 of the body member The conductive clips 56 and 60 which contact the conductive clips 55 and 58 on the upper and lower end plates 12 and 14, respectively, are supported on the ends of the valve block 18 and, as shown in FIG. 11, the ends of the coil 17 are also secured to these members so that the coil and valve block are connected in parallel.

At the bottom end of the arrester the contact 56 of the lowermost coil assembly 16 makes contact with the bottom cap of the housing of the arrester (not shown) and thence is connected to ground. At the top of the arrester the connecting member 60 of the uppermost coil assembly 17 of the arrester similarly makes contact with the top cap of the housing of the arrester and thence is connected to the line, thus completing the electrical circuit as shown in FIG. 2. It will be noted that there are no series valve blocks interposed between the arrester and ground or between the arrester and the line.

As a guide to those skilled in the art and as an example of a successful arrester, certain design details and dimensions of a lightning arrester for a 765 kv AC transmission system are given below.

The discs are composed of permeable ceramic material having a porosity of 18 percent. The diameter of the arcing chamber is 7% inches. The longer fingers 45 project inwardly about 1% inches from the periphery of the arcing chamber and the shorter fingers 46 project inwardly about nine of an inch. The spacing between the plates is about 0.08 inch except in the vicinity of the troughs where the spacing is increased to 0.14 inch. The arcing chamber occupies about 270 of the perimeter of the discs; the electrodes are composed of flat copper about 0.06 inch in thickness; the gap at the sparkover region 26 is 0.08 inch and the radius of curvature of the electrodes beginning at the sparkover region is about 1% inches which is reduced to about 1 inch where the curved edges 27 merge into the straight portions 28. The coils have a mean diameter of about 8 inches and contain 50 turns of No. 14 copper wire.

. SUMMARY OF OPERATION The curves constituting FIG. 3 illustrate the electrical characteristics of an arrester embodying the present invention when subjected to a severe switching surge. In the following discussion, which is based on the same assumptions as FIG. 3, a description is given of the events that occur in one of the gap units of an arrester having the desired 1,200 kv sparkover when the arrester is subjected to a severe switching surge, the arrester being constructed as set forth above.

The pre-ionizing gap is arranged to spark over at a lower voltage than the main gap. Thus, typically the pre-ionizing gap 37 would be adjusted to spark over at about 4.7 kv. This provides electrons to give more consistent breakdown and sparkover voltage for the main gap which will spark over at approximately 6 kv, the series of 198 gaps in the arrester providing the required sparkover voltage of not more than 1,200 kv.

Sparkover takes place at the zone 26 where the electrodes 24 and 25 approach each other most closely, as indicated by the line a in FIG. 4. As soon as sparkover has taken place, the gap arc voltage drops to a very low value, the curves of FIG. 3 applying to individual gap units as well as to the total arrester except that the voltage values given in FIG. 3 are many times greater than for a single gap unit.

As soon as the arc has been established it begins to move outwardly along the curved surfaces of the electrodes 24 and 25. The are is caused to move because of the shape of the electrodes and by the magnetic field that is generated by the coil; the field strength increases rapidly as soon as the wave front has passed and as the coil current increases, as indicated by the curve. The are moves outwardly under the influence of the magnetic field through the zones indicated by dotted lines b and c, the arc voltage building up as the arc is extended and as energy is absorbed from the are by the contact of the arc with the porous walls of the arcing chamber. The arc continues to move through the zone indicated by line d where it begins to contact the tips of the fingers 45 and finally the shorter fingers 46. Of course, contact may not occur simultaneously at all of the fingers. The field continues to act upon the arc and the arc plasma, thus further lengthening the are as shown by the path e. As the field strength increases with time, the arc may move still farther between the fingers, as indicated by path f. The porous, permeable gap plates permit hot gases to escape ahead of the arc.

The dimensions of the fingers and their spacing, the depth of the arc chamber and the magnetic field strength are chosen so that when the arc is extended between the fingers, as shown by the lines e and f, for example, the voltage drop in the extended portions of the are between tips of adjacent fingers becomes so large that the arc tends to restrike and re-establish itself along the original path d across the tips of adjacent fingers. It will be noted that the arc, when it is forced around the ends of the fingers, can still see itself which gives it a greater tendency to restrike. As soon as the arc restrikes between adjacent tips of the fingers, it again is moved outwardly by the action of the field. This action may take place at some or all of the finger tips and probably does not act at the same time or in synchronism on all of the finger tips. However, there is a very rapid fluttering of the are between positions such as shown by paths d, e and f, and this fluttering action keeps repeating itself and thus distributes the heating and erosion activity of the are over the rather large area of the arc chamber between the path d and the path f. This action differs fundamentally from prior arresters in which the arc is driven to a final stopping position against a wall where there is a high local heating and erosion level.

Thus, instead of the stable are that was sought for and secured in prior types of arresters, the arcs in the gap units of the present invention are unstable. Not only does the fluttering take place between the adjacent fingers, but also the fluttering and consequent change in arc voltage results in rapid movement of the ends of the arc along the surface of the electrodes and fluttering also takes place between the ends of the shorter fingers 46a and 46b and the relatively straight adjacent surfaces of the electrodes as indicated by line 3. This action prevents the ends of the are from remaining stationary near the ends of the straight portions 28 of the electrodes and reduces the chance of damage to the electrodes that might otherwise occur.

It is because of the fluttering action that the arc gaps are able to withstand prolonged surges without serious damage, since the heating is distributed over substantial areas and localized hot spots are eliminated. However, restriking does not take place at the sparkover zone, probably for the reason that the fluttering and restriking action takes place in comparatively short sections of the arc, and the arc voltage for the total length of the arc does not vary excessively as a result of the fluttering.

Consistency of operation is improved by the troughs or gutters 49 and 50 surrounding the fingers and particularly the portions of these gutters around the inner ends of the fingers. The troughs function to increase the spacing between the adjacent gap plates 13a and 13b in this area, as shown particularly in FIG. 8. Increasing the spacing decreases the absorption of energy and transfer of heat from the arc to the plates in these areas. In service, the porosity of the tips of the fingers is reduced by the heating that is concentrated at these points. However, because the troughs reduce the effectiveness of the area adjacent the tips from a heat and energy absorption standpoint, the decrease in porosity in the material adjacent the tips does not materially affect the overall operation of the gaps because the areas adjacent the tips are made initially less effective than the remaining areas of the gap. While the troughs are shown as extending along the sides of the fingers to the periphery of the plates, their primary effectiveness is near the tips of the fingers.

As the arc moves outwardly in the arcing chamber and flutters between the fingers, as shown by lines e and f, the arc voltage increases until, in the example given it may reach almost 6 kv per gap unit, producing a total arrester voltage of between 1,000 and 1,200 kv as shown in FIG. 3. As soon as the surge is dissipated, this gap voltage, which is greatly in excess of the system crest line to ground voltage, is sufficient promptly to shut off the arc.

It will be noted that as shown in FIG. 3 the initial voltage drop across the parallel-connected coil and valve block assemblies is so large that if this voltage drop were added to the maximum voltage ultimately developed by the arcs, the total voltage across the arrester would exceed the allowable protective level of 1,200 kv. However, with the arrester of the present invention the coil and valve block voltage drcreases so rapidly with respect to the rate of increase of the arc voltage that the total of the gap arc voltage and the coil and valve block voltage is always substantially less than the allowable protective level. This result is accomplished in the present arrester by the correlated design of the gap units and the coil assemblies. In the present arrester, by the time that the gap arc voltage is increasing rapidly, say after 200 microseconds, the coil and valve block voltage has been reduced toward the low value that results primarily from the resistance of the copper in the coils 17.

The gap units are much larger than gap units of currentlimiting arresters heretofore employed. For example, in wellknown and successful arresters embodying current-limiting gaps and porous ceramic discs manufactured by leading manufacturers of lightning arresters in the United States, the

ing the gap units in the present arrester are about 8 /2 inches in diameter and the diameter of the arcing chamber itself is only slightly less than 8 inches. These dimensions are required because of the heavy currents of 1,800 amperes or even more that the arresters are required to handle. Such currents require a relatively large radius of curvature preferably at least 1 inch on the electrodes and greater depth of arcing chamber than arresters in lower voltage systems that are subjected to smaller currents.

The sparkover region is near the center of the arcing chamber and the diverging curved portions of electrodes are disposed on opposite sides of a diameter of the plates. The diverging curved portions each extend in.a continuous curve from the sparkover region across a second diameter of the plates at right angles to the first diameter and then back to the same side of the second diameter, terminating in the slightly diverging straight portions 28. The are in the present arrester must be moved outwardly a substantial distance from the sparkover region approximately 3 inches in the present example between the uninterrupted faces of the discs before it reaches the ends of the fingers 45 and 46. The first part of this movement is relatively slow because the field created by the coils 17 is initially relatively weak, and the, arc voltage increases at a relatively slow rate as indicated by the first part of the curve of gap arc voltage in FIG. 3. As the field builds up the curve of gap are voltage becomes steeper and the voltage rapidly reaches its maximum value.

During the time that the arc is moving relatively slowly (actually the movement is very rapid since the maximum arc voltage is developed in about 500 microseconds as shown by the curve) the coil and valve block voltage is being rapidly reduced. This is accomplished by making the coils with such a small number of turns thatthe inductance of the coils is low and their time constant correspondingly low. Also, the maximum arc voltage per gap unit with these large discs is about 6 kv, which is several times greater than the maximum arc voltage of any prior type arresters known to me. It is desirable to have the maximum arc voltage high, not only so that the power follow current will be shut off promptly after a surge has'passed, but also to limit the current flowing through the arrester during prolonged surges, thus reducing the likelihood of damage to the arrester.

l-leretofore, in arrester design it has been the practice to seek constructions that provide stable arcs and eliminate restriking of arcs. According to the present invention, instability of the arcs near the periphery of the gap units is sought after to obtain the desired fluttering action that distributes heating effects of the are over substantial areas, thus maintaining a high rate of heat exchange between the arcs and the adjacent porous surfaces as well as reducing concentrated areas where erosion might take place. This, again, is made possible by the present design in which the fingers 45 and 46 project inwardly from the periphery ofthe arcing chamber more than an inch and in which the bases of the fingers are about an inch apart, whereas the tips are only about five eighths inch apart. Thus, as the arc is forced outwardly between adjacent fingers, as shown by linefin FIG. 4, the actual length of the arc from the tip of one finger down into the space, across the space and back to the tip of the adjacent finger frequently is from 2 to 3 inches, whereas the space between the hot tips is only a fraction of these amounts. Thus, the voltage drop of the portion of the arc in the spaces between the fingers becomes great enough that the arc restrikes readily between tips of adjacent fingers, producing the desired fluttering action.

The same sort of action takes place between the electrode surfaces 28 and adjacent tips of the fingers 46a and 46b as shown by the line g, with the result that not only are the erosion and heating effects distributed over substantial areas of the gap plates 13, but also the arc is prevented from standing still on the electrodes so that electrode overheating and erosion is greatly reduced. All of these factors contribute to the durability and efficiency of lightning arresters embodying the present invention.

While the foregoing summary relates particularly to the operation of the arresters in an extra high voltage AC line, it is to be understood that arresters made according to the invention and, in fact, arresters made in accordance with the detailed description given herein also are capable of successful and advantageous operation in high voltage DC systems. As those skilled in the art understand, different grading circuits are preferred for DC systems and the number of gap units should be selected to provide a sparkover voltage not greater than the allowable protective level of the system. However, tests have shown that arresters such as disclosed herein have the ability to protect 450 kv DC systems, this being the highest DC system voltage presently known to me.

The fluttering action of the spark in the gap units is particularly advantageous for DC systems, since it minimizes local overheating during prolonged surges. With prior types of arresters in which the arc is caused to impinge against a peripheral wall and remain there, the heating that takes place may result in such a large reduction in the arc voltage that the arrester loses its ability to interrupt the power follow current in a DC system in which the system voltage and current do not go to zero as they do in AC systems. Arresters made according to the present invention are not subject to this difficulty and hence have the ability to interrupt power follow current in DC systems, and in AC systems before the system voltage reaches zero.

From the foregoing it will be evident that the invention provides improved lightning arresters and gap units that are especially adapted for severe service such as in extra high AC systems and high voltage DC systems where very heavy currents must be interrupted and where prolonged surges are likely to be encountered. The arresters are durable, and reliable and can be produced at reasonable cost when the duties they are required to perform are taken into consideration. These advantages flow from the gap design which enables the gaps to withstand prolonged surges and to develop high are voltages and from the correlation of the gap design with the coil assemblies which makes it possible to utilize the high are voltage to shutoff power follow current without requiring the use ofseries valve blocks and also without developing voltages in excess of the allowable protection level in the system.

Those skilled in the art will appreciate that various changes and modifications may be made in the specific embodiment of the invention disclosed herein without departing from the spirit and scope of the invention. The essential characteristics ofthe invention are set forth in the appended claims.

I claim:

1. A lightning arrester comprising a plurality of gap units connected in series and in series with at least one coil assembly, each coil assembly comprising a coil for creating a magnetic field for extending the arcs in the gap units and a valve block in shunt with the coil, the series connected gap units and coil assembly(ies) being adapted to be connected between a line of the system to be protected and ground without the intervention of any series connected valve block, the gap units being of the current limiting type in which substantial arc voltages are developed as the arcs are extended in them, the total sparkover voltage of the gap units being below the allowable protective level of the system, the gap units being capable of developing a total maximum gap arc voltage in excess of the system line to ground voltage crest and less than the allowable protective level, and the coil assembly(ies) having such resistance and inductance characteristics (1) that it (or they) provide a substantial initial voltage drop immediately after sparkover, the said voltage drop across the coil assembly(ies) immediately after sparkover when the arrester is subjected to a high voltage surge having a steep wave front being of a magnitude that if added to the total maximum gap arc voltage developed in the gap units would exceed the said allowable protective level, the initial flow of current through each coil assembly being substantially entirely through the valve block thereof, (2) that the total voltage drop across the coil assembly(ies) immediately after sparkover gradually decreases as the total gap arc voltage increases as the arcs are extended in the gap units under the influence of the magnetic field(s) developed by the coil assembly(ies), the portion of current flowing through the coil of each assembly gradually increasing and the portion of the current flowing through the valve block thereof gradually decreasing after sparkover until substantially all of the current is flowing through the coil(s) at the time the voltage across the coil assembly(ies) reaches its minimum value. and (3) that the rate at which the total gap arc voltage increases after sparkover and the rate at which the voltage drop across the coil assembly(ies) decreases after sparkover are such that the total voltage across the arrester, comprising the sum of the total voltage drop across the coil assembly(ies) and the total gap arc voltage, does not exceed the allowable protective level.

2. A lightning arrester according to claim 1 adapted for service in extra high voltage AC systems and having a sparkover voltage of about 1,200 kv, the gap units being capable of developing a total gap arc voltage in excess of 1,000 kv.

3. A lightning arrester according to claim 1 in which the gap units comprise superposed porous permeable ceramic plates defining arcing chambers between them and a pair of electrodes in each arcing chamber, the peripheral portions of the arcing chambers being closed and being provided with inwardly extending fingers between which the arcs flutter.

4. A lightning arrester according to claim 3 in which there is a trough in one of the plates around the inner end portions of the fingers, the troughs increasing the axial thickness of the arcing chamber in the zones around said inner end portions.

5. A lightning arrester according to claim 3 in which the electrodes have curved arcing surfaces that diverge from each other from a sparkover region near the center of the arcing chamber, the radius of curvature of said surfaces being at least 1 inch through at least 90 ofcurvature.

6. A lightning arrester according to claim 5 in which the curved arcing surfaces of the electrodes merge into substantially straight surfaces that extend substantially to the periphery ofthe arcing chamber.

7. A lightning arrester according to claim 6 in which the inwardly projecting fingers are spaced circumferentially around the periphery of the arcing chamber from a zone adjacent the substantially straight surface of one electrode to a zone adjacent the substantially straight surface of the other electrode, whereby fluttering of arcs can take place between said substantially straight surfaces and the fingers nearest said substantially straight surfaces.

8. A gap unit for lightning arresters and the like comprising two plates having oppositely disposed faces spaced apart to provide an arcing chamber, a pair of electrodes in said chamber, the electrodes having edge surfaces disposed close to each other and on opposite sides of a diameter of the plates to provide a sparkover region within the arcing chamber and diverging portions extending from the sparkover region, a plurality of circumferentially spaced fingers extending inwardly from the periphery of the arcing chamber toward the electrodes, the fingers blocking the arcing chamber against circumferential movement of an arc while permitting radial movement of an arc between the fingers, the length and spacing of the fingers being such that an are extended into the space between two fingers as by an electromagnetic field becomes unstable and restrikes between the tips of the fingers, then is moved radially again and again restrikes and so on, whereby the arc flutters rapidly and distributes heating and erosion effects on the plates throughout a substantial area.

9. A gap unit according to claim 8 in which there is a trough in one of the plates around the inner end portions of the fingers, the troughs increasing the axial thickness of the arcing chamber in the zones around said inner end portions 10. A gap unit according to claim 8 wherein the electrodes are spaced from the adjacent surfaces of the plates by layers of insulating tape adhesively secured to the faces of the electrodes.

11. A gap unit according to claim 8 wherein the diverging portions of the electrodes are curved with a radius of curvature ofat least 1 inch.

2. A gap unit according to claim ll wherein the plates are composed of permeable ceramic material.

13. a gap unit according to claim 12 wherein the arcing chamber is circular and has a diameter of about 7% inches, and the fingers in the zone opposite the curved portions of the electrodes project radially inwardly about I inch and are spaced apart at the periphery of the plates by a distance of about 1 inch and at their tips by about inch.

l4..A gap unit according to claim 13 wherein the curved portions of the electrodes merge into and terminate in substantially straight runner portions that extend substantially to the periphery of the discs, there being shorter fingers extending inwardly from the periphery of the discs toward said substantially straight portions.

15. A lightning arrester for extra high voltage AC service and high voltage DC service comprising a series of gap packs, each gap pack being made up of a plurality of gap units connected in series and in series with at least one coil assembly, each coil assembly comprising a coil for creating a magnetic field for extending the arcs in the gap units and a non-linear resistor in shunt with the coil, the series-connected gap units and coil assemblies being adapted to be connected between a line of the system to be protected and ground without the intervention of any series connected valve block, the gap units being of the currentlimiting type of which substantial arc voltages are developed as the arcs are extended in them and the coil assemblies providing a substantial initial voltage drop immediately after sparkover, which voltage drop thereafter decreases, the total sparkover voltage of the gap units being below the allowable protective level of the system, the gap units being capable of developing a total are voltage in excess of the line voltage crest and ofa magnitude that if added to the initial voltage drop across the coil assemblies when the arrester is subjected to a high voltage surge having a steep wave front would exceed the said allowable protective level, the gap units and the coil assemblies being designed so that the gap arc voltage increases at a rate not substantially greater than the rate at which the voltage drop across the coil assemblies decreases after sparkover until such time that the voltage drop across the coil assemblies has reached a value that when added to the maximum gap arc voltage would not exceed the designed protective level, each gap unit comprising two circular rigid permeable ceramic plates having interfitting peripheral portions for retaining the plates in co-axial relationship, the plates having oppositely disposed faces spaced apart to provide an arcing chamber, a pair of electrodes in said chamber, the electrodes having edge surfaces disposed close to each other and on opposite sides of a diameter of the plates to provide a sparkover region, diverging curved portions having a radius of at least 1 inch, the diverging curved portions each extending in a continuous curve from the sparkover region across a second diameter of the plates at right angles to the first diameter and then back to the same side of the second diameter as the sparkover region and terminating in substantially straight runner portions that extend substantially to the periphery of the discs, a plurality of circumferentially spaced fingers extending radially inwardly from the periphery of the discs toward the electrodes, the fingers blocking the arcing chamber against circumferential movement of an arc while permitting radial movement of an are between the fingers, the length and spacing of the fingers being such that an are extended into the space between two fingers becomes unstable 7 and restrikes between the tips of the fingers, then is moved radially again and again restrikes and so on so that the arc flutters rapidly and distributes heating and erosion effects on the plates throughout a substantial area, 7

16. A lightning arrester according to claim 15 wherein the distance from the tip of one finger outwardly along the finger to the base thereof adjacent the periphery of the arcing chamber to the base of the next adjacent finger and inwardly along said adjacent finger to the tip thereof is at least four times the shortest distance between the tips of said fingers. 

1. A lightning arrester comprising a plurality of gap units connected in series and in series with at least one coil assembly, each coil assembly comprising a coil for creating a magnetic field for extending the arcs in the gap units and a valve block in shunt with the coil, the series connected gap units and coil assembly(ies) being adapted to be connected between a line of the system to be protected and ground without the intervention of any series connected valve block, the gap units being of the current limiting type in which substantial arc voltages are developed as the arcs are extended in them, the total sparkover voltage of the gap units being below the allowable protective level of the system, the gap units being capable of developing a total maximum gap arc voltage in excess of the system line to ground voltage crest and less than the allowable protective level, and the coil assembly(ies) having such resistance and inductance characteristics (1) that it (or they) provide a substantial initial voltage drop immediately after sparkover, the said voltage drop across the coil assembly(ies) immediately after sparkover when the arrester is subjected to a high voltage surge having a steep wave front being of a magnitude that if added to the total maximum gap arc voltage developed in the gap units would exceed the said allowable protective level, the initial flow of current through each coil assembly being substantially entirely through the valve block thereof, (2) that the total voltage drop across the coil assembly(ies) immediately after sparkover gradually decreases as the total gap arc voltage increases as the arcs are extended in the gap units under the influence of the magnetic field(s) developed by the coil assembly(ies), the portion of current flowing through the coil of each assembly gradually increasing and the portion of the current flowing through the valve block thereof gradually decreasing after sparkover until substantially all of the current is flowing through the coil(s) at the time the voltage across the coil assembly(ies) reaches its minimum value, and (3) that the rate at which the total gap arc voltage increases after sparkover and the rate at which the voltage drop across the coil assembly(ies) decreases after sparkover are such that the total voltage across the arrester, comprising the sum of the total voltage drop across the coil assembly(ies) and the total gap arc voltage, does not exceed the allowable protective level.
 2. A lightning arrester according to claim 1 adapted for service in extra high voltage AC systems and having a sparkover voltage of about 1,200 kv, the gap units being capable of developing a total gap arc voltage in excess of 1,000 kv.
 3. A lightning arrester according to claim 1 in which the gap units comprise superposed porous permeable ceramic plates defining arcing chambers between them and a pair of electrodes in each arcing chamber, the peripheral portions of the arcing chambers being closed and being provided with inwardly extending fingers between which the arcs flutter.
 4. A lightning arrester according to claim 3 in which there is a trough in one of the plates around the inner end portions of the fingers, the troughs increasing the axial thickness of the arcing chamber in the zones around said inner end portions.
 5. A lightning arrester according to claim 3 in which the electrodes have curved arcing surfaces that diverge from each other from a sparkover region near the center of the arcing chamber, the radius of curvature of said surfaces being at least 1 inch through at least 90* of curvature.
 6. A lightning arrester according to claim 5 in which the curved arcing surfaces of the electrodes merge into substantially straight surfaces that extend substantially to the periphery of the arcing chamber.
 7. A lightning arrester according to claim 6 in which the inwardly projecting fingers are spaced circumferentially around the periphery of the arcing chamber from a zone adjacent the substantially straight surface of one electrode to a zone adjacent the substantially straight surface of the other electrode, whereby fluttering of arcs can take place between said substantially straight surfaces and the fingers nearest said substantially straight surfaces.
 8. A gap unit for lightning arresters and the like comprising two plates having oppositely disposed faces spaced apart to provide an arcing chamber, a pair of electrodes in said chamber, the electrodes having edge surfaces disposed close to each other and on opposite sides of a diameter of the plates to provide a sparkover region within the arcing chamber and diverging portions extending from the sparkover region, a plurality of circumferentially spaced fingers extending inwardly from the periphery of the arcing chamber toward the electrodes, the fingers blocking the arcing chamber against circumferential movement of an arc while permitting radial movement of an arc between the fingers, the length and spacing of the fingers being such that an arc extended into the space between two fingers as by an electromagnetic field becomes unstable and restrikes between the tips of the fingers, then is moved radially again and again restrikes and so on, whereby the arc flutters rapidly and distributes heating and erosion effects on the plates throughout a substantial area.
 9. A gap unit according to claim 8 in which there is a trough in one of the plates around the inner end portions of the fingers, the troughs increasing the axial thickness of the arcing chamber in the zones around said inner end portions
 10. A gap unit according to claim 8 wherein the electrodes are spaced from the adjacent surfaces of the plates by layers of insulating tape adhesively secured to the faces of the electrodes.
 11. A gap unit according to claim 8 wherein the diverging portions of the electrodes are curved with a radius of curvature of at least 1 inch.
 12. A gap unit according to claim 11 wherein the plates are composed of permeable ceramic material.
 13. a gap unit according to claim 12 wherein the arcing chamber is circular and has a diameter of about 7 3/4 inches, and the fingers in the zone opposite the curved portions of the electrodes project radially inwardly about 1 inch and are spaced apart at the periphery of the plates by a distance of about 1 inch and at their tips by about 5/8 inch.
 14. A gap unit according to claim 13 wherein the curved portions of the electrodes merge into and terminate in substantially straight runner portions that extend substantially to the periphery of the discs, there being shorter fingers extending inwardly from the periphery of the discs toward said substantially straight portions.
 15. A lightning arrester for extra high voltage AC service and high voltage DC service comprising a series of gap packs, each gap pack being made up of a plurality of gap units connected in series and in series with at least one coil assembly, each coil assembly comprising a coil for creating a magnetic field for extending the arcs in the gap units and a non-linear resistor in shunt with the coil, the series-connected gap units and coil assemblies being adapted to be connected between a line of the system to be protected and ground without the intervention of any series connected valve block, the gap units being of the current-limiting type of which substantial arc voltages are developed as the arcs are extended in them and the coil assemblies providing a substantial initial voltage drop immediately after sparkover, which voltage drop thEreafter decreases, the total sparkover voltage of the gap units being below the allowable protective level of the system, the gap units being capable of developing a total arc voltage in excess of the line voltage crest and of a magnitude that if added to the initial voltage drop across the coil assemblies when the arrester is subjected to a high voltage surge having a steep wave front would exceed the said allowable protective level, the gap units and the coil assemblies being designed so that the gap arc voltage increases at a rate not substantially greater than the rate at which the voltage drop across the coil assemblies decreases after sparkover until such time that the voltage drop across the coil assemblies has reached a value that when added to the maximum gap arc voltage would not exceed the designed protective level, each gap unit comprising two circular rigid permeable ceramic plates having interfitting peripheral portions for retaining the plates in co-axial relationship, the plates having oppositely disposed faces spaced apart to provide an arcing chamber, a pair of electrodes in said chamber, the electrodes having edge surfaces disposed close to each other and on opposite sides of a diameter of the plates to provide a sparkover region, diverging curved portions having a radius of at least 1 inch, the diverging curved portions each extending in a continuous curve from the sparkover region across a second diameter of the plates at right angles to the first diameter and then back to the same side of the second diameter as the sparkover region and terminating in substantially straight runner portions that extend substantially to the periphery of the discs, a plurality of circumferentially spaced fingers extending radially inwardly from the periphery of the discs toward the electrodes, the fingers blocking the arcing chamber against circumferential movement of an arc while permitting radial movement of an arc between the fingers, the length and spacing of the fingers being such that an arc extended into the space between two fingers becomes unstable and restrikes between the tips of the fingers, then is moved radially again and again restrikes and so on so that the arc flutters rapidly and distributes heating and erosion effects on the plates throughout a substantial area.
 16. A lightning arrester according to claim 15 wherein the distance from the tip of one finger outwardly along the finger to the base thereof adjacent the periphery of the arcing chamber to the base of the next adjacent finger and inwardly along said adjacent finger to the tip thereof is at least four times the shortest distance between the tips of said fingers. 